Voice operated control arrangement for telephone cable systems



Dec. 22, 1931.

G. CRISSON ET AL VOICE OXERATED CONTROL ARRANGEMENT FOR TELEPHONE CABLE SYSTEMS Filed Sept. 10,- 1929 Vzilume +33 2 Sheets-Sheet 1 London level Z f0 W INVENTORS Dec. 22, 1931. G. CRISSON ET AL 1,837,316 VOICE OPERATED CONTROL ARRANGEMENT FOR TELEPHONE CABLE SYSTEMS 7 Filed Sept. 10. 1929 2 Sheets-Sheet 2 ATTORNEY OFF Patented Dec. 22, 1931 UNITED STATES NEW JERSEY, ASSIGNORS TO AMERICAN TELEEHON AND TELEGRAPH. oomrnnm,

A CORPORATION ,OF NEW YORK l PATENT OFFICE GEORGE CRISSON, OF EAST ORANGE, AND SUMNER 3'. Warsaw, or soon! E GE,

" voIoE OPERATED CONTROL ARRANGEMENT FOR TEnEPHonE CABLE SYSTEMS Application filed September 10, 1929. Serial No. 391,598. V

This invention relates to telephone transmission systems', and moreparticularly to transmission systems involving along section of submarine cable.

In order that wire telephone transmission may take place between points separated by a large body of water, as, for example, is the case in telephoning between New York, and London, it is necessary that atleast one "longsection of submarine cable be incorporated in the transmission circuit. Because of the expense of constructing such a cable, it is impractical to provide two cables for fourwire operation, and therefore the one long section of submarine cable, for example, be-

tween Newfoundland and Ireland, mustbe used for two-way transmission. It is, however, quite feasible to operate on a four-wire basis those portions of the circuit which exftend between the ends of the cable and the terminal switching stations at say New York and London, even thoughparts of the'four wire circuit must be carried in shorter sections of submarine cable as, for example,

in the 'Irish Sea and in the short stretch of waterbetween Nova Scotia and NQWfOlIIId'. land; These relatively short cables may be worked on the two-way plan or the transmission paths for opposite directions may be '1 separated by using carrier transmission, In

any case substantial echoes in the four-wire to causenoise, it will at least have a noise level determined by theresistance noise (or circuits may be caused by the intervening lengths of submarine cable.

WVith reference to the long section of submarine cable, which is operated on a two-way basis, however, the attenuation is at best enormous, and although such a cable is relatlve- 1y free from external disturbing influences 7 noise due to'thermal agitation) of the cable.

In order, therefore,-that the currents received after transmisslon over the cable have suflicient volume to be above this noise level, the

voice energy must be applied at the sending end of the cable ata very high Volume level, so that the difference in level between the sending energy and the received energy at a given end of the cable is'very much greater L thanis the case with any typeof telephonesystem heretofore: employed. Thissituation introduces.- transmission problems of an order. never heretofore encountered.

With this great disparity between applied and received energy at the ends of. a cable, it is obviously necessary that voice. operated switching arrangements be-provided v for prefor suppressing: echoes. necessary to locate the voice. operated atrangement adjacent the ends of, the: cable.

This is forthe reasoni that the receiving amplifier must be very sensitive in. order to'step the very weak attenuated currentsincoming from the cable up to a normal volume for venting singing in the four-wire circuitandJ Furthermore,;1t 1s,

On the other hand,- to locate,.at the endsof the cable, al-lof the apparatus fOrsuppressing singing and echoesd'n; the .four-Wire-" circuit is inadvisable, as: it is impracticable to: maintain-at the relatively isolated: points at. which: the cable terminates the; technical operating, supervisory and maintenance'staff necessaryqto control apparatus-' for the. various conditions obtained by different: telephone extensions at the switching tenninah It is therefore proposed to arrangeithe mainnvoice operated equipment for the suppressionof; singing at the-terminal. stations, for eX-ample,'New York and London, and tofso arrange the equipmentfat; the cable: terminals that thevoicepoperated mechanisms at the cable terminals need not. require continuous inspection and adjustment. Forthis reason the voice-operated: devices at the cable ends will be referred to as auxiliary devic'es, even though they may'consist ofmore-compl'ex circuits than: the main devices. To accomplish thisresult, the voice, operated equipment at the switching terminals is arranged so as to normally disable the transmitting side of the circuit to prevent singing, the disabling means being removed in response to the voice currents. Arrangements are also provided for disabling the transmission path at the switching terminal in response to voice cur rents received over the receiving side of the four-wire circuit., Since the transmitting side of the circuit is disabled except when used for talking, the voice-operated equipment at the cable terminal is protected from noise currents arising in the connecting circuits beyond the switching terminal, and, as the fourwire circuit to the cable terminal will be laid in telephone cable, the four-wire circuit itself should be relatively free from noise. By providing volume adjusting means at the switching terminal to keep the voice currents applied to the-transmitting path at a normal level well above noise, the voice operated apparatus at the cable terminal may be made very-sensitive and need not be carefully adjusted to discriminate between signals and noise. 7

By making the voice operated apparatus at thecable terminals highly sensitive, such apparatus will respond to the initial part of the voice wave before such-wave approaches its peak value. Therefore, the'delay circuit ordinarily used to prevent clipping, when the sensitivity is held to a lower value due to noise in the connections, may be made very small or even omitted. This is important from the standpoint of the time of transmission between the voice operated appara tus at one terminal of the cable and that at the other, as the delay circuit time is added to the time of transmission over the cable.

It is desirable to keep down the time of transmission between the two sets of echo suppressing apparatus which ultimately determine which subscriber has control of the circuit in order to reduce the possibility of a lock-out. When two full echo suppressor equipments are associated with a circuit at the same point, the possibility of the subscribers mutually locking each other out is limited to the .case where the voice currents,

from both subscribers arrive at the echo suppressors at the same instant. If, however, the two full echo suppressor equipments are moved toward the terminals of the circuit so that a definite time is required for transmis sion between the two set-s of equipment, mutual lock-out can take place when the voice currents of the two subscribers arrive at the corresponding echo suppressors within a period equal to the time of transmission be- ,tween the two sets of suppressors.

In the case of the submarine cable, it is evident that the lock-out time cannot be reduced to a period less than the time of transmission over the cable, for the reason that the two sets of echo suppressor equipment cannot be closer together than the ends of the cable, and to the time of transmission over the cable the delay time of the delay circuits must be added in determining the lockout time. Hence, the reduction of the delay due to the delay circuits is important.

As already stated, voice operated devices for the suppression of singing are to be lo cated at the switching terminals. If these arrangements were so designed that the talking subscribers voice currents in passing over the transmitting path of the four-wire circuit could disable the receiving path at the switching terminal, the look-out time would be the time of transmission from one switching terminal to the other, and, as the fourwire circuits leading from the cable terminals to the switching terminals might be of considerable length, and noise conditions are such that delay circuits are necessary for proper operation of the transmitting relay, the increase in lock-out time might be so considerable as to make the circuit very unsatisfactory from the subscribers standpoint. Accordingly, it is proposed to arrange the voice-operated apparatus at the switching terminals so that only the transmitting path of the four-wire circuit is controlled, and no means is provided at the switching terminals to disable the receiving path. This will give control of the circuit from the standpoint of lock-out to the voice-operated devices at the cable terminal and thus reduce the look-out time accordingly.

Another factor which must be considered in the telephone system involving a long submarine cable is the so-calledrelaxingcurrent. The characteristic impedance of the cable will involve a considerable reactance component. lVhen voice currents of large energy are applied to one end of the circuit for transmission to the other end, energy is stored up in the reactance of the cable, and, when the voice potential ceases, a unidirectional gradually decaying current flows out of the cable because of the energy stored up in its reactance. Since the reactance is large. and the applied energy must necessarily be large. (because of the great attenuation of the cable), this relaxing current will. at the instant when the applied; energy ceases, be large and may require a considerable time before it decays to a value small enough to cause no disturbance in the sensitive receiving apparatus associated with the cable at its terminal. This relaxing current may therefore be for a considerable period of time very much greater in magnitude than the'signaling currents which are to be received over the cable, and, if nothing is done to reduce its effect, the voice-operated relays for disabling the receiving path of the four- Wire circuit at the cable terminal might have to be provided with a hang-over much longer than that necessary to take care of the echoes lit returned from the cable. i It is. therefore proposed, in accordance with the presentinvenout of both the cable and'its balancing network, and these relaxing currents will tend to cancel each other to the extent that. the cable and network, balance each other, so that the resultant componentof relaxing cur- 7 rent in the receiving path will be accordingly reduced.

Other features of. the invention will be clear from the following detailed description thereof when read in connection with the accompanying drawings, in which Figure-l is a symbolic diagram showing the general layout of a telephone'system employing as one link long submarine cable; FigsJQ and 3 are curves'showing the volume levels at the various points of the circuit in the two direc tions of transmission, respectively; Fig. 4 is a schematicdiagram of the apparatus at the switching teri. inal and at the cable terminal. respectively; F g. 5 shows a series of curves illustrating certain features of the invention, and Fig. 6 a simplified circuit for explain ing the effect of resistance noise in the circuit.

The general layout of atypical system in accordance with the invention is shown in Fig. 1. In interpreting this figure, it should be borne in mind that transmission paths comprising two wires, one acting as return for the other, ,or one wire with a ground return, are symbolically represented by a single line and that amplifiers are repre sented by rectangles with arrows .to indicate the direction of transmission. The echo suppressing devices are symbolically represented by a line attached directlyto the voice c1rcui't which'operates the echo suppressor, with an arrow at the-other end adjacentthe circuit which is'to-becontrolled. The open or a submarine cable between Newfoundland and Ireland, and with the terminals of the submarine cable connected to they switching terminals by four-wire circuits each comprising a one-way transmitting circuit for talking'in the one direction and a one-way receiving circuit for talking in the opposite direction. The principalunit of the system is the transatlantic submarine cable indicated at X in Fig. 1 Each end of the cable is balanced by networks symbolically indicated at J 2 and J and the ends of the cable are also connected to the four-wire circuits comprisingpaths TL and RL- extending from Newfoundland to New York, and paths TL and BL extending from Ireland to London. The dotted lines used in the four-wire circuits indicate that the conductors areof the land line type, of standard form'equipped with repeaters and all necessary apparatus although it will be understood that parts of the four-wire circuit will pass through undersea cables in order to span the Irish Sea and that portionof the Atlantic Ocean lying between Nova Scotia and Newfoundland. It should also be understood that the layout of Fig. 1 merely typifies the general principle .and that the particular locations of apparatus will in practice be varied according to the submarine conditions that'exist between the two switching terminals of any given circuit.

'' At the New York switching terminal, the transmitting line TL is normallydisabledat E and a voice operated mechanism operating in response to voice currents applied to the circuit from the terminal line L defeats the disabling means so that the transmission line TL becomes operative. is provided to enable the voice operated device E to operate before the voice currents arrive atthed1sabl1ng points. A voice op erated device A is associated with the re ceiving line BL and operates in response to voice currents received over the line RL to disable the transmitting path. A volume regulator VH is also provided at the New York terminal to bring the speech currents to a definite strength for transmission over the system. The apparatus at the London terminal is similar to that at New York, corresponding elements being designated by the same reference characters primed. It will be noted that no voice operated device is pro-1 a voice operated device E is associated with the transmitting path to make the circuit operative in response to transmitted voice currents. Another voice operated, device H controlled from the transmitting path is arranged to disable the receiving path RL. A delay circuit C is provided to enable the devices and H to perform their functions before the voice currents arrive at the point where the transmitting line TL is disabled.

A delay circuit C its Another voice operated device A, is associated with the receiving path RL for disabling the transmitting path TL in response to the voice currents received over the submarine cable. An amplifier D is associated with the transmitting line TL for bringing the voice currents up to a very high volume before applying them to the cable, whose attenuation is very great. The voice currents received over the cable are, of course, greatly attenuated during transmission and are stepped up to a desired level by means of an amplifier G in order to operate the voice operated device A An additional amplifier M may be provided where desired. The apparatus a the Ireland end of the cable is similar to that at Newfoundland.

Figure 2 is a volume level diagram to indicate the volume levels at the (lilierei'it points of the circuit in transmitting from New York to London, while F 3 shows a similar volume level diagram for transmission in the opposite direction. Since volume control. arrangements VB, and VR are used at the New York and London terminals, so that the transmitting volume applied to the system is always substantially constant, the diagrams of Figs. 2 and 3 are so drawn as to show the voice volume, referred to zero reference volume, at each point of thesystem. In considering Figs. 2 and 3. it snould be borne in mind that the Zero reference volume does not represent sero energy, but, on the contrary, represents an arbitrarily chosen volume level which may be roughly described as the average amount of electrical energy produced by the average subscriber in talking over a long distance circuit into an ordinary telephone system using a standard transmitter. The unit of volume used in plotting the diagram is the decibel, and the number of decibels indicated at any point in the diagram is a function of the ratio of the voice energy at the point indicated to the amount of energy arbitrarily chosen as the zero level. Suppose, for example, a current I, is applied to a circuit and a current I is received at the receiving end, the number of decibels representing the gain in transmission over such circuit will be equal to 20 log If, therefore, the ratio of the received current to the applied current is unity, so that there is no change in energy, the change in transmission will. be zero decibels. the received current is ten times as great as the applied current, the gain in transmission will be 20 decibels. If, on the other hand, the received current is of the applied current, the change in transmission will he 20 decibels. The following table gives the number of decibels'corresponding'to certain current ratios,

the current ratios being given in the first column:

g i Decibels g g 1 0 4O 1O 1 0 2O 10 3. 1 6 1 O l O 2 6 4 ,/2 3 2 1 O 1 1 1 J2 71 3 2 l 1 2 6 4. 316 10 1 0 1O 2O 1O 1O 4O 1 O" The corresponding energy ratios are given in the third column. It will be noted that the energy ratio corresponding to any number of decibels will be the square of the corresponding current ratio. From this table it will be seen that doubling the energy represents a gain of 3 decibels, while reducing the received energy by one-half represents a loss of 3 decibels. Doubling the current, on the other hand, represents a gain of 6 decibels, while halving the current, represents a loss of (3 decibels. These particular values should be noted, as they are of some importance in understandin the effect of the hybrid coils and balancing networks, as will be described later.

Owing to the necessary use of voice operated switching apparatus, the system is arranged to deliver a substantially constant volume of speech to the connecting line at either end, regardless of the volume impressed upon the other end of the system. This is accomplished by providing the volume regulating apparatus at the switching terminals so that, regardless of the loudness of the speech applied to the system from the connecting line, the volume applied to the transmitting line of the four-wire circuit will be constant. As will be seen from the curves of Figs; 2 and 3, the volume delivered at the receiving end of the system is 20 db. weaker than the zero or reference volume. Under normal conditions with reference volume applied to either end of the system, and 20 db. delivered at the other end, the transmission loss between terminal o'llices is 20 db. When stronger or weaker volumes are applied at either end of the system, the gain will be adjusted by the velume regulator at the switching terminals, and, if necessary, by the volume regulator at the cable terminal, to bring the volume to the normal value at the voice operated apparatus, in the submarine cable, and at the far end of the system. The range of applied volume to be provided for is from +10 to -30 db, with respect to reference volume at the switching terminal. This is shown, for

example, by the twobranches of the volume level curves at the extreme left in Fig. 2

and at the extreme right in Fig. 3. The sys-' applied to the system, it drops to .83 db.

upon passing through the hybrid coil. This is because the applied energy divides at the hybrid coil between the transmitting line and the'receiving line, due to the conjugate arrangement of the connection, and only half of the energy is ultimately delivered to the transmittin line In the table above given, halving the energy resulted in a loss of 3 decibels, which accounts for the condition just described. I

By means ofthe volume regulator VH1, the volume level finally applied to-the'transmitting line at the switching terminal may be reduced (or raised) to +5 db. This value is chosen as the maximum constant volume which may be transmitted over the system without producing excessive cross-talk in adjacent land lines. Y

The volume level at the .cableterminal, for

example, at Newfoundland, is again brought up to 5 db. by means of the line repeaters at that point, the volume adjuster VB be-' ing operated,ifnecessary, to correct for any deviation fromgthe desired level. The volume level actually impressed upon the submarine cable should be made as high as possible with due regard to the limitations imposed by the loading material, in order that the cost of the cable may bekept down by designing it for a'large transmission loss without permitting the received signal to fall too near the noise level in the cable. cordingly, the amplifier D may raise the level to +33 db. The energy, upon passing the hybrid coil at the terminal of the cable, loses 3 decibels because only half of the energy goes to the cable, the other half being distributed to the balancing network J. Accordingly, decibels are applied to the cable. Assuming that the attenuationof the cable is such as to reducetheapplied energy 150 decibels, it will be received at theoppm site terminals of the cable ata level of -120 db. and will. be reduced 3 db. further by reason of the hybrid CQll arrangement at the other end cit-the cable. The extremely low volume thus received is again raised to +5 db. by therepeaters M and G's and then.

transmitted over the receiving path RL of ihe tour-wire circuit to the London switching terminal. The transmission in the opposite direction, as shown by the curve of Fig.3, will be similar to that described. It will be understood, of course, that the values of volume level above given are merely illustrative and that in any practical design the volume levels may be :changed very considerably from those described. 7 1

The volume regulating apparatus at the switching terminals may merely consist'of a potentlometer associated with a repeater which is adjusted by a technical operator who is guided by a volume indicator; but, if desired, it may be an automatic arrangement. In any event, its function is to keep the volume-of speech at all parts of the four-wire circuit at a normal value except for minor variations so that sufficient energy will available to operate the voice operated apparatus at the cable terminals. While the lines of the four-wire circuit will be provided with pilot wire-transmission regulation, there may be some residual variations in the transmission loss of the four-wire circuit.. Such variations may be taken care of by the volume regulating apparatus, such asv YR at the cable terminal. The variation to be expected at this point will be a slow variation with changes in the transmission eiliciencyof the four-wire circuit, and. consequently the volume adjustment at the cable terminal need not be fairly continuous asisnecessary at the switching terminal, where differences between theloudness withawhich difierent subscribers speak, and the effective distances of the subscribers from the terminal, must be taken care of. The transmission loss' of the submarine cable is, of course, enormous,but since thecable is exposed to very constant temperature con ditions, it should not be necessarytomake provision for automatic or quick adjustment of the gains of the receiving amplifiers associat'ed with the cable. If any serious changes occur, they are likely to take place slowly and can be compensated for by adjustments made at the time of the lining-up tests of the circuit. Of course automatic devices might be used'here if found necessary.

7 As has beenpreviously stated, the great transmission loss or" the submarine cable and the desirability .of reducing the inherent lock-out and breaking difiiculties of the system. to the lowest possible degree make it necessary-"to have auxiliary voice 'operated anti singing and an echo suppressing apparatus at the endsof the cable itsef. Certain advantages are secured, however, by locating the-principal anti-singing voice operated devices and delay circuits at the switching terminals (New York and London), as compared to havingthe main voice operated devices at the ends of the cable,,with the primary control of volume levels also exercised atthe switching terminals." This enables the auxiliary voice operated devices at the cable terminals-to be" operated-fat relatively high sensitivity. The receiving voice operated relay at the cable terminal need have a hangover only long enough to take care of echoes due to unbalances between the control station and the end of the cable. Echoes from the connected circuits are suppressed by the suppressor A at the terminal oflice whose hangover can be adjusted as required. As no transmitting echo suppressing relay is included at the switching terminals to control the receiving side of the circuit, incoming speech from the transatlantic cable takes control of the circuit even though outgoing speech is actually traveling through the land cable. Consequently, while the over-all delay of the circuit is slightly increased by the use of the additional voice operated devices, smaller delay circuits are required in connection with the auxiliary voice operated devices at the cable terminals, and, as lockouts occur between the apparatus at the cable terminals, the probability of mutual lock-out is substantially limited to that due to the time of transmission through the cable. By locating the principal anti-singing voice 0perated devices at the switching terminals, the technical operators at those terminals will be immediately aware of the necessity for adj us ment in relay sensitivity necessitated by clipping or excessiveland line noise, and are in a position to make decisions and issue instructions to the switching operators in case troubles are caused by faulty connecting circuits.

With this general idea of the operation of the system in mind, the details of the apparatus at the switching terminal and the cable terminal, as shown in Fig. 4: may be readily understood.

It may be assumed that Fig. 4 shows the apparatus at New York and Newfoundland, the corresponding Landon and Ireland installations being in all respects similar.

Referring to Fig. 4, the connecting line L at the switching terminal is balanced by a network N and is associated by means of a hybrid coil 10 with the transmitting line TL and receiving line RL of the four-wire circuit. The transmitting line includes a line amplifier L of ordinary construction, a volume regulator VR which may be simply an amplifier with an adjustable potentiometer operated either manually or automatically, and a second line amplifier D The volume regulator VR is for the purpose of maintaining the volume level of volce currents in the transmitting line TL at a substantially constant value so that variations in the loudness with which a given speaker talks, or

C for the purpose of enabling the relay E to cut through the line TL before the voice currents which operate the amplifier-detector arrive at the point where the line TL is disabled.

The receiving line RL includes amplifiers G and M and is provided with a voice operated arrangement comprising an amplifierdetector B and control relay A for disabling the transmitting line TL when voice currents are received from the line RL. If necessary, a delay circuit P may be provided to enable the relay A to operate before echoes from the received voice currents pass through the hybrid coil 10 into the transmitting circuit. The relay A should control the transmitting line TL ahead of the volume regulator VH so that if it is automatically operated, it will not be adjusted by echoes passing from the receiving circuit through the hybrid coil into the transi'nit-ting circuit.

At the cable terminal, the cable X is balanced by a network J and, by means of a hybrid coil I is associated with the transmitting line TL and the receiving line BL oi the four-wire circuit. Owing to the enormous dilierence in transmission level between the sending energy from the transmitting circuit and the receiving energy transferred from the cable to the receiving line, shielding is provided to protect the low level parts of the receiving circuit from high level transmitting apparatus. This is indicated by the dotted line rectangles in the figure, which surround certain portions of the transmitting and re ceiving equipment, respectively.

The transmitt-in g line includes a line amplifier L an auxiliary volume or transmission regulator VH and a transmitting amplifier l) for bringing the final volume to be applied to the cable up to the desired level indicated by the diagram of Fig. 2. Normally, the transmitting line is disabled just ahead of the hybrid coilbymeans eta relay E which is controlled by an amplifier-detector arrangement F 2 bridged across the transmitting line ahead of a delay circuit C which may be provided for the purpose of enabling the relay E to operate before the voi e currents arrive at the point where the circuit is disabled. Since the amplifier-detector F may be made very sensitive, due to the fact that noise currents are kept off of the transmitting line at the switching terminal, except when voice currents are being transmitted. the delay circuit C may be made very small and indeed may be omitted entirely without causing undue clipping.

It will he noted that the contacts of the relay E are bridged across the output circuit of the amplifier D rather than across the input circuit. so that any noise arising within the amplifier will be prevented from reaching the cable and the receiving side of the system.

The amplifier-detector F also controls a relay H by means of which the receiving line 'RL maybe disabled. Normally, the receiving circuit is operatively connected over the back contact of the relay H but when the relay operates, the circuit of the line BL is broken as soon as the armature leaves its back contact. The receiving line includes a low power amplifier G and a high power amplifier Gr for bringing thevery weak received currents in the caole up to a desired value for transmission over the receiving line .RL. An, attenuation equalizer. K maybe connected between the two ampl fiers; for

equalizing the attenuation of the currents transmitted over the cable, the cable, of. course, having the effect of attenuating the high frequencies to a. much greater extent than the low frequencies. Anainplifier-detector arrangement B is associatedwith the receiving line and controls a relay A for disabling the transmitting line TL. The contact controlled by the relay A is preferably associated with theline TLahead of the volume regulator VR in order. that where an automatically adjusted volume regulator is used, it will notice adjusted by echo currents.

The operation will now be described briefly. Waves originating at a subscribers telephone come in-over theconnecting line L of Fig. 4 and are raised to the proper volume at the terminal oiliceand operate thea'nti-singing relay IE at that point, so that they may be passed through the delay circuit 0 and thence over the line TL to the cable terminal. Upon arriving at the cable ter minal office, they operatethe amplifier-de tector F5 and the relaysE and H and then pass through the amplifier-D 'to the cable. The delay apparatus C gives the relayll time to operate before the Wave reaches its contacts, thus avoiding initial clipping. This voice operated apparatus obviously must opcrate on all voice waves, since failure of the relay E to operate would prevent the transmission of speech, whilefailure of there lay H to'operatewould permit powerful echoes to pass into the receiving line and operate the relay A thus interrupting the outgoing speech. o

The anti-singing relay E atthe terminal oiiice insures that noise from the subscribers telephone and any terminal' conn'ecting cir quiet. Forthis reason, theamplifier-detecplifier D torF may be made asinuch as 20 db. more sensitive than would be the case if the antisinging relay E were not used at the terminal office. Furthermorathe delay introduced at 0 may be made very small, or, in

fact, the delay circuit may be omitted entirely, as the amplifiendetector,being very sensitive, ill respond to the initial weak part of'a telephone wave before it approaches its peak, so that therelay E willbe operated before any important: part of the telephone wave is cut off. 7 I

The auxiliary volume regulator VH is provided to take care of-thc residual variations of the transmission loss of the line TL between the switching terminal and the cable terminal. This may be used" to correct the volume of the speech waves to +5 db. before they reach the input terminals of the am- V Since the voice volume is regulated'atthe terminal oflice, VR 'need not n cessarily: be a volume regulator, but may.

be a transmission regulator controlled'by a i ilotcurrent transmitted from the terminal office when speech is not passing.

When waves come in over the cable X, they pass, without hindrance, through the receiving amplifier. G ,Ian equalizer K and high G toward the terminal power amplifier office, provided the relay is not operated. j

In passing, these waves operate the amplifierdetcctor B and the relay A which latter short circuits the transmitting side of the circuit and so prevents operation of the .anti

sin 'in rela E and the echo su res'sor l b b l relay H either by voice or by echoes during the passage of incoming waves. Later, the waves reach the terminals of the amplifierdetector B and. operate the relay A which short-circuits the transmitting circuits and prevents operation of the anti-singing relay E Thesewaves finally pass to the connectinglineL. V v I It is notessential thatthe relay A operate in response to very. Weak received speech jwaves,provided the transmission loss fronr the receiving to the transmitting side of the four-wire circuit through theunbalance of the hybrid coil at the switching terminal is great enough to prevent the operation of the if relay E 'by the echoes of waves that are too weak to operate the relay A and that the transmission loss through any intermediate imbalances is great enoughftoprevent the" operation of the relays and H; by such echoes. Relay E will be very sensitive, however, as pointed out above, so that if intermediate;unbalances exist it will be necessary for relay A to be correspondingly sensitive. If" no; intermediate. unbalances occur, it will be suiiicient to make A, sensitive enei'ighso that it will respond to waves the echoes ofiwhici will operaterelay E since E; and IL will not operate until E has operated. i V

"i so The relay A is completely protected from land line and terminal noise when speech is not being received over the cable. The line amplifier M prevents noise coming back from the receiving line BL to operate the relay A while at the distant end of the cable the anti-singing relay corresponding to E prevents any noise from entering the cable at hat terminal when speech is not passing, and consequently the only noise coming from that direction to which the re lay A is subject-ed is that arising in the submarine cable itself and in the apparatus such as the amplifier G equalizer K etc. This noise will be due principally to thermal agitation, and, after passing through the equalizcr K will be much stronger at high frequencies than at low frequencies. The amplifier-detector B: should therefore be most sensitive to the important voice frequencies and should be quite. insensitive to the higher voice frequencies, thus reducing the liability of the relay A being operated by thermal agitation to a considerable extent.

In order to ensure full operation of relay A it is desirable for the speech waves in the receiving side RL of the four-wire. circuit to be of the order of 30 db. stronger than the noise which exists when no speech is being received. This noise, of course, includes thermal agitation in the network and oflice apparatus and in the cable, as well as interference picked up by the cable itself. However, the interference picked up by the cable should be very small, as there are no power lines or other sources of interference in the ocean, and this type of interference would therefore be limited to points near the ends of the. cable.

The line amplifiers L and M are ordinary one-way repeaters which serve as terminal repeaters for connecting the four-wire circuit to the voice apparatus. The repeater L in the transmitting side serves to raise the level of the transmission from whatever the line delivers to +5 db. (or other desired level) which is suitable for operating the apparatus associated with the transmitting side of the system. The amplifier M performs a similar function for the receiving side, changing the level from whatever level is furnished by the receiving amplifier G's to +5 db. or such other level as it is desired to impress upon the receiving side of the four-wire circuit. Since the amplifier G: should be normally adjusted to provide this level. at its output, the function'of the repeater M is essentially to provide a one way device to prevent the amplifier-detector B from operation by echoes or noise coming back over the line BL from the direction of the switc ing terminal. This is particularly important if there are no other one-way repeaters in the receiving side of the fourwire circuit between the nearest hybrid coil 1.56 db. at high frequencies.

and the voice operated apparatus B In such a case, voice waves from the switching terminal could reach the input terminals of the amplifier-detector B if the amplifier M were not used.

Owing to the large transmission loss of the submarine cable, which may be assumed to be in the neighborhood of 150 db. at the upper end of the voice range (about 3000 cycles), certain precautions must be taken that ordinarily are not necessary. As has been previously pointed out, the amplifier D will supply, in its output circuit, speech waves at about 33 db. above reference volume, of which 30 db. is impressed upon the cable while 3 db. is lost in the hybrid coil due to the fact that half of the energy flows into the balancing network J On the other hand, the incoming waves from the submarine cable will reach the amplifier G at about 123 db. below reference volume .l20 db. being received from the cable and 3 db. lost in the hybrid coil due to the fact that half of the energy is transmitted into the transm tting circuit where it is wasted. The total difference of level is therefore about Consequently, if the circuit from the output of the amplifier D to the hybrid coil I is not perfectly balanced longitudinally, an unbalanced or longitudinal voltage will act through the system and transfer noise to the cable even when the circuit is short-circuited by the relay E This will take place regardless of the loss to normal transmission caused by the relay E the longitudinal current flowing in effect through the capacity between the windings of the hybrid coil. This longitudinal effect can be 'reatly reduced by placing a shield grounded to the cable sheath between the line and drop windings of the hybrid coil. A similar shield, connected to the office ground, should also be provided between the windings of the output transformer (not shown) of the amplifier D The effect of these shields is to shunt to ground the displacement current which tends to pass through the capacity of the windings. Also, it is desirable that the secondary winding of the output transformer of the amplifier D and the drop winding of the hybrid coil be made in two closely balanced and closely coupled parts with grounds at the midpoints. The series resistances of the conductors connecting the amplifier AD to the hybrid coil should also be accurately balanced.

In order that the relay IL may disable the receiving circuit as quickly as possible, it is arranged to open the receivin circuit as soon as its armature leaves its back contact. In order to prevent transmission through the capacity between its armature and back contact, it is provided with a front contact so connected as to short-circuit the input of the amplifier G It is undesirable to open both sides of the receiving circuit at the contacts of the relay H as a momentary unbalance would occur if thetwo contacts didnot open simultaneously, and this might produce enough disturbance to cause false operation of the'relay' A therebyclipping speech out-- going from the transmissionline TL to the cable X. In orderto make possible theuse of this unsymmetrical arrangement of the relay H the transformer. N is provided, the transformer-also serving to insulate the cable conductor froml the inputcircuit of the amplifier Gr and to shieldsaid amplifier from the longitudinal voltages on the conductors- The hybrid coil I performs its usual function of separating the outgoing signals from the incoming signals. In the case of the sub marine cable,however,- it performs an even more important function in that it assists in reducing the relaxing current from the cable. The loaded submarine cable has a characteristic impedance which is not a pure resistance but involves a very considerable reactive component. As has been previously stated, the voice currents are applied to the.

cable from the line TL at a very high volume level, and the voicepotentials thus store up in the rea'ctance of the cable a very large volume of energy. with the result that, when the applied voicepotential ceases, the energy stored up in the reactance causes a gradually decaying uni-directionalcurrent to flow 'outof the cable. This current, at the moment of cessation of voice waves, is so large in a magnitude that it mi ht produce very serious l the network at the same timethat the re lasing current flows out of the cable, and both relaxing currents will decay'at substantially thesame rate, so that they tend to annul each other with respect to any 'efiect in the receiv ing circuit BL. Assuming ,thatthe' balance between thecable and the network is 40'db.,

. that is, that the unbalancecurrent is 40 db.

weaker than the current entering the network,

. not counting losses due to the division of. energy by the hybrid coil,,the relaxing current from thenetwork should balance that from the cable to at least this degree. Hence, the unbalanced relaxing current would be so small P that excess. hang-over above that necessary to take care of echoes many not be required. In fact, the reduct on of relaxing current should be even greater than this because the balance between'the networlcand the charac teristic impedance of the lineupon which the relaxing current depends'would be greater.

energy is reflected back due to irregularities in the cable, with the result. that the current measured at theinput at anyfrequency may. be greater or less than- 'should .beexpected .fromthe characteristic impedance, depending uponwhether the reflected current is in phase or not. The result is that {the sending end impedance, while in a general way following thecharacterlstm mpedance, w llhave humps or undulations. The resistance component ofthe sending end impedance would be as shown by the thin lineof Fig. 5. The degree or balance between the cable and the network at any frequency is determined by the difference at that frequency between the dotted curveof-Figfb and} the fine line curve of said figure and the corresponding differences between the reactance'curves .(not shown).

Such imbalances include the effect of echoes reflected from irregularities in the cable. The

balance, with respect to relaxing currents,

however, is not affectedby reflected currents, and it depends upon the rea'ctancecomponent simulate the'terminal impedance of the'cable as indicated by the fine line of Fig.5, and,-

for that reason, the network is designed to simulate the characteristic impedance, s nce the irregularities in the terminal impedance deviate about equally .in both dl-rectlons above and'below the characteristic impedance.

Consequently, 'due to the closer simulation between the impedance of'the network and. the characteristic impedance of the cable, than that which exists between the'impedance of the network and the 'terlninalimpedanc'e of. the cable, the balance will be considerably betterfrom the standpointfof the relaxing current than in the caseof echo currents.

{The use of the hybrid coil I (Fig bf course, involves certain penalties. One of these is that'the transmitting amplifier D ":1

must ,raise'the power level 3' (1b., that is,

it must supply twice the power which would f be required if the amplifier wereconnected directly with a submarinec ablm Thisis for the reason that one-half the slgnalingenergy is wasted in the balancing network. Another penalty is that the margin between the voice waves coming from the cable and the noise due to thermal agitation in the cable, in the network, and in the associated apparatus at the input terminals of the amplifier G is reduced by 3 db. With respect to any ther' mal noise arising in the receiving circuit beyond the bridge connection to the hybrid coil, it is evident that this reduction in man gin occurs since such thermal noise is not aifected by the hybrid coil, while half of the signal energy coming from the cable is wasted by being transmitted into the transmitting channel. While at first thought it would seem that the hybrid coil should produce no change so far as the margin between the sig nal and the thermal noise in the cable is concerned, actually the margin is effectively reduced in this case, also. That this is so, will be apparent when it is remembered that while the presence of the hybrid coil reduces the thermal noise of the cable by one-half, the network J which would not be present if the hybrid coil were omitted, adds an equivalent amount of thermal noise, so that in effect the total amount of noise due to thermal agitation remains unchanged as a result of the introduction of the hybrid coil, but the received signal is reduced by 3 decibels. Since the thermal noise, after'passing the equalizer K is strongest at high frequencies, this margin could be restored to some extent by reducing the upper limit of the frequency band which is received and utilized.

As has been previously stated, the matter of shielding is of considerable importance, since the hybrid coil and associated apparatus sometimes work at a high volume level and sometimes at a low volume level. Owing to the very low level at the input of the receiving amplifier G this amplifier and its associated wiring should be shielded from the high level parts of the circuit. Accordingly, this amplifier, together with the equalizer K and the input wiring of the amplifier, are enclosed withina grounded'shield indicated schematically in dotted lines in Fig. 4. Similarly, the high level parts of the circuit, including the wiring leading from the output of the transmitting amplifier D to the hybrid coil, the hybrid coil itself, and the network J are included within another grounded shield schematically represented in dotted lines in the figure. The high power amplifier Gr need not be shielded if it is not located too close to the output of the transmitting amplifier D There will generally be an appreciable difference in potential between ground at the cable terminal station andthe armor of the submarine cable which is in effect grounded through the seawater. It is therefore desirable that shields between the windings of the hybrid coil I and the transmitter N3 be connected to the armor of the submarine cable in order that the potential difference between the shield and the windings connected to the cable conductors be as small as possible. On

the other hand, there should be no connection between the office ground and the cable armor, as this would disturb the potential of the armor with respect to the cable conductors and tend to increase the cable noise. One end of the secondary winding of the transformer N must necessarily be connected to the filament of the first vacuum tube of the amplifier G It is also essential that the shield associated with the transformer N be connected to this point because any potential difference between the shield and filament would cause noise in the input circuit of the receiving amplifier G Therefore, at least the first stage of this amplifier, and preferably'all stages of the amplifier, should be pro vided with isolated and shielded batteries for the vacuum tubes of these stages, so that they need not be connected to the cfiice ground. This will reduce the potential difference between the low level circuits of the amplifier G and the cable conductors to the lowest possible value and so reduce the possibility of trouble due to stray capacity around the shield.

Due to the extremely low level of the signal currents received from the cable, precautions must be taken to keep thermal noise in the system to a minimum. Mention has already been made of the effect of the hybrid coil and network in reducing the margin between incoming speech waves and the noise due to thermal agitation in the cable, the network and input circuits of the amplifier G The thermal noise in the cable itself is determined by the design of the cable, and, so far as the present problem is concerned, must be treated as unavoidable. Certain precautions may be taken in laying out the input arrangements and early stages of the receiving amplifier G to avoid unnecessary thermal noise.

In ordinary telephone repeater circuits, it is necessary that the circuit provide a fairly good termination for the line with which it works, in order to avoid reflected waves, and consequently the bridged terminals of the hybrid coil, in the case of a SEQ-type repeater, are connected to a resistance which gives the desired termination, and the drop across this resistance is applied through the transformer to the grid of the first stage vacuum tube. For example, in a SEQ-type repeater system, the input transformer corresponding to N of Fig. l would have a resistance bridged across its primary winding. If this resistance is eliminated as it is in the arrangement disclosed in Fig. 4:, the voltage due to the voice wave impressed upon the grid of the first vacuum tube would be doubled. Re-

ferring to the chart previously given, it will be seen that this doubling of the applied volt- .the temperature and resistance.

age, and consequently of the output current of the amplifier G has the effect of increasing the output level of the amplifier G by 6 db. The voltage due to thermal agitation, on the other hand, would only increase by and hence the amplified thermal noise in the output of the amplifier G will only be increased 3 db. I I

In order to understand this, let us refer to the diagram of Fig. 6, in which R may be taken to be the resistance bridged across the primary winding of the transformer N and R the equivalent resistance component of the impedance of the network J hybrid coil L and cable X. In accordance with the practice in repeater design, B would be made equal to R Now, the thermal noisevoltageis a random varying voltage whose effective value is the same for all frequencies and is a function of It may be expressed by formula V where T is the temperature, R the resistance and K a constant. At absolute zero tem perature, there is no thermal noise.

Suppose that with R disconnected the voltage due tothe resistance noise of R as measured at V is 6. Now, if a resistance equal to R but having zero temperature (and hence no noise voltage) is bridged across the circuit at R the voltage drop at V due to resistance noise from R w1ll be Likewise if R 'is at ()lfllllll temoerature and resistance equal to R at zero temperature is substitutedfor R the voltage drop at V due to the resistance noise from R will be i Now, if both R and R are in circuit and are at normal temperature, the voltages combins, but as they are random varying voltages they do not combine additively but as the square root of'the sum of their squares. Hence, the term nal voltage at V will be Hence, since thevoltage is ewith RN removed,

itfollows that the current-will increase by ofthermal noise within the smallest est echo.

wave voltage produced in E the terminal voltage at V with R removedmay be'expressed by the value c. Vllen the resistance R is brid ed across the circuit, however, the term nal voltage at V Wlll be reduced to any reflection from the terminal will have been so attenuated by the time it reaches the other end that it is negligible. Furthermore, the provision of the voice operatedapparatus at the terminal insures thatthe wave reflected'from thereceiving apparatus would have no appreciable effect upon the transmission. Consequently, the resistance across the primary winding of the transformer N should be omitted, thereby increasing the differential between voice currents and resistance noiseby3db. v

' The attenuation equalizer K will, of course, include resistance elements which would add some thermal noise to the circuit. It is necessary to provide this equalizer to compensate for the attenuation frequency curve of the cable, but the effect ofits thermal noise may be decreased by connecting it beyond the amplifier G By doing this,

the incoming voicewaves will be raised to a relatively high level before passing through the equalizer, and the thermal noise dueto the equalizer will therefore be relatively insignificant.

The windings of the hybrid coil I and-0f the input transformer N should be so designed as to introduce as little resistance as possible inorder to keep their contribution possibl limits. V

The amplifier-detector 13-2 and the relay A constitute one-half of anordinary echo suppressor. It is suflicient that the relay A operate before any echoes arising in the fourwire circuit reach its contacts. Its hangover should be long enough to catch the slow- B A at the switching terminal, which takes care of any echoes from connectlng circuits, it should not be necessary for relay A 1 to hang over longer than the roundtrip time to the switching terminal office and back.

The relay H mustoperate as fast as possible and slightly'before any waves passing the contacts of the relay E can reach those of the relay H This will prevent the high power wave from the amplifier D from reaching the amplifier G It may not be Owing to the use of equipment necessary for the relay H to complete its travel and short circuit the input of the amplifier G before the relay E operates.

The relay E should operate as fast as possible in order to avoid initial clipping with little or no artificial delay in the delay circuit C but its operation should not be so "fast that the relay H cannot stop all waves passing the contacts of E The relay E should also hang over long enough to avoid terminal clipping. T he relay H on the other hand, must hang over long enough so that echoes coming back from the cable, due to outgoing speech and noise from the transmitting line TL, will have ceased, and for the relaxing current from the cable to fall to a low enough value to prevent :talse operation of the relay A It will be obvious that the general principles herein disclosed may be embodied in many other organizations widely different from those illustrated without departing from the spirit of the invention as defined in the following claims.

lVhat is claimed is:

1. In a telephone system, a pair of termi' nal switching stations, a tour-wire circuit comprising a transmitting line and a receiv ing line extending between each terminal station and an auxiliary station, a transmission medium between said auxiliary stations. means at said terminal switching stations to establish connections between the system and connecting lines, means at said terminal switching stations to prevent the transmission of noise from connecting lines over the transmitting lines to said auxiliary stations at all times when said transmitting lines are not being used for voice transmission, means associated with said transmitting lines at each auxiliary station to disable the associated receiving line when the transmit ting line is transmitting voice currents to said transmission medium, and means with the receiving line at each auxiliarv station to disable the associated transmitting line when telephone currents are being received from said transmission medium.

2. In a. telephone system, a pair of terminal switching stations, a four-wire circuit comprising a transmitting line and a receiving line extending between each terminal station and an auxiliary station. a transmission medium between said auxiliary stations, when at said terminal switching stations to establish connections betwcei the system and connecting lines, means at said ter minal switching stations to prevent the trans mission of noise from connecting lines over the transmitting lines to said auxiliary stations at all times when said transmitting lines are notbeing used for voice transmission, means associated with said transmitting lines at each auxiliary station to disable the asso ciated receiving line when the transmitting line is transmitting voice currents to said transmission medium, said last-mentioned means being sensitive to the weak initial portions of a voice wave before it approaches its peak value, and means associated with the receiving line at each auxiliary station to disable the associated transmitting line when telephone currents are being received from said transmission medium.

3. In a telephone system, a pair of terminal switching stations, a four-wire circuit comprising a transmitting line and a receiving line extending between each terminal station and an auxiliary station, the lines of said four-wire circuits being laid in cable so as to be relatively free from noise due to external sources, a transmission medium between said auxiliary stations, means at said terminal switching stations to establish connections between the system and connecting lines, means at said terminal switching stations to prevent the transmission of noise from connecting lines over the transmitting lines to said auxiliary stations at all times when said transmitting lines are not being used for voice transmission, means associated with said transmittin lines at each auxiliary station to disable the associated receiving line when the transmitting line is transmitting voice currents to said transmitting medium, said last-mentioned means being sensitive to the weal; initial portions of a voice wave before it approaches its peak value, and means associated with the receiving line at each auxiliary station to disable the associated transmitting line when telephone currents are being received from said transmission medium.

L. In a telephone system, a pair of terminal switching stations, a four-wire circuit comprising a transmitting line and a receiving line extending between each terminal station and an auxiliary station, a transmission medium between said auxiliary stations, means at said terminal switching stations to establish connections between the system and connecting lines, means at said terminal switching stations to normally disable said transmitting lines, thereby preventing the transmission of noise thcreover from connecting lines, voice-operated means associated with said transmitting lines for rendering them operative, means associated with said transmitting lines at each auxiliary station to disable the associated receiving line when the transmitting line is transmitting voice currents to said transmission medium, and means associated with the receiving line at each auxiliary station to disable the associated transmitting line when telephone currents are being received from said transmission medium.

5. In a telephone system, a pair of termin al switching stations, at four-wide circuit comprising a transmitting line and a receiving line extending between each terminal station and'an auxiliary station, a transmission medium between said auxiliary stations, means at said terminal switching stations to establish connections between the system and connecting lines, means at said terminal switching stations to normally disable said transmitting lines, thereby preventing the transmission of noise thereover from connecting lines, voice-operated means associated with said transmitting lines for rendering them operative, means. associated with said transmitting-lines at each auxiliary station to disable the associated receiving line when the transmitting .line is' transmitting, voice currents-to said transmission medium, said last-mentioned means being sensitive to the weak initial portions ofa voice wave before it approaches its peak value, and means associated with the receiving line at each auxiliary station to disable the associated transmitting line whentelephone currents arebeing received from said transmission medium.

6; In a telephone system, a pcair of terminal switching stations, a four-wire circuit comprising a transmitting line and a receiving line. extending between each terminal station and an auxiliary station, the lines o'f'said four-wire circuits being laid in cable so as to be relatively free from noise from external sources, a transmission medium between said auxiliary stations, means at said terminal switching stations to, establish connections between the system and connecting lines, means at said terminal'switching stations to normally disable said transmittingclines, thereby preventing the transmission of noise :thereover from connecting lines, voice-operated means associated with said transmitting lines for rendering them operative, vmeans associated with said transmitting lines vat each auxiliary station to disable the ;associated receiving line when the transmittingline is transmitting voice currents to said transmission medium, said "last-mentioned means being sensitive to the weak initial portions of 1 a voice wave before it approaches its peak value, and means associated w th the receiving line-at each auxiliary station to disable the associated transmitting .line when telephone currents are being received fromsaidtransmission medlum. V I

' I t l l t f t l. n a eiep lone sys em, a palr o ermi .nal switching stations, a four-wire circuit comprising a transmitting line and a receivingline extending between eachtermmal stationand an auxiliary station, a transmission medium between said auxiliary stations, means at said terminal switching stations to establish connections'between the system and connecting lines, means at said terminal switching stations to normally disable said transmitting lines, thereby preventing'hthe transmission of noise thereover from connecting lines, voice-operated means associated with'said transmitting lines for rendering them operative, means associated with the receiving line at each terminal switching station to disable the corresponding transmitting line when voice currentsiare being received, means associated with said transmitting lines at each auxiliary stationtto disable the associated receiving line when the transmitting line is transmitting voice currents to said transmission medium, and means associated with the receiving line'at each auxiliary'station to disable the associated transmitting line when telephone currents V are being received from; said transmission medlum.

8. 111 a telephone system, a pair of terminal switching stations, a four-wire circuit comprising a transmitting line and a receiving line extending between each terminal station and an auxiliary station, a transmission mediumbetween said auxiliary stations, means at said terminal switching stations to establish connections between the system and connecting lines, means at said terminal switching stations to normally disa blesaid transmitting lines, thereby preventing the transmission of noise thereover from connecting lines, voice-operated means a sociated with said transmitting lines for rendering them operative, means'associated with the receiving line at eachterminal switch;

proaches its peak value, and means asso- V ciated with the receiving line at each auxiliary station to disable the associated trans.-

-mitting line when telephone currents are being received from said transmission medium.

9. In a telephone system, a pair of terminal switching stations; a four-wire circuit comprising a transmitting line and a receiving line extending between each terminal station and anauxiliary station, the lines oiisaid four-wire circuits being laidin cable so as to be relatively free from noise from external sources, a transmission medium between said auxiliary stations, means at said terminal, switching stations 'toestablish con ncctions betw-een the system and connecting lines, means at said terminal switching .stations to normally. disable said transmitting lines, thereby preventing the tran'smiss'ionol? inoise thereover from connecting lines, voice operated meansassociated with said transmitting lines for-rendering them operative,

meansiassociated with the receiving line at each terminal SWlllClllIlgStfitlQll to disable the corresponding transmittingflline when l the transmitting lines to said auxiliary minal switching stations, a four-wire circuit comprising a transmitting line and a receiving line extending between each terminal station and an auxiliary station, a transmission medium between said auxiliary stations, means at said terminal switching stations to establish connections between the system and connecting lines, means at said terminal switching stations to prevent the transmission of noise from connecting lines over stations at all times when said transmitting lines are not being used for voice transmission, means associated with said transn itting lines at each auxiliary station to disable the sociated receiving line when the transmitting line is transmitting voice currents to said transmission medium, said last-men tioned means being sensitive to the weal; initial portions of a voice wave before it approaches its peak value, means at each terminal switching station to transform the voice current volume received from a connecting line to a substantially constant predetermined volume so that the oice currents will arrive over the transmitting lines at the auxiliary stations with substantially constant volume, and means associated with the receiving line at each auxiliary station to disable the associated transmitting line when I telephone currents are being received from said transmission medium.

11. In a telephone system, a pair of terminal switching stations, a four-wire circuit comprising a transmitting line and a receiving line extending between each terminal station and an auxiliary station, a transmission medium between said auxiliary stations, means at said terminal switching stations to establish connections between the system i and connecting lines, means at said terminal switching stations to normally disable said transmitting lines, thereby preventing the transmission of noise thereover from connecting lines, voice-operated means associated with said transmitting lines for rendering them operative, means associated with said transmitting lines at each auxiliary station to disable the associated receiving line when the transmitting line is transmitting voice 3 currents to saio transmission medium said last-mentioned means being sensitive to the weak initial portions of a voice wave before it approaches its peak value, means at each terminal switching station to transform the voice current volume received from a connecting line to a substantially constant predetermined volume so that the voice currents will arrive over the transmitting lines at the auxiliary stations with substantially constant volume, and means associated with the receiving line at each auxiliary station to disable the associated transmitting line when telephone currents are being received from said transmission medium.

12. In a telephone system, a pair of terminal switching stations, a four-wire circuit comprising a transmitting line and a receiving line extending between each terminal station and an auxiliary station, a transmission medium between said auxiliary stations, means at said terminal switching stations to establish connections between the system and connecting lines, means at said terminal switching stations to normally disable said transmitting lines, thereby preventing the transmission of noise thereover from connecting lines, voice-operated means associated with said transmitting lines for rendering them operative, means associated with the receiving line at each terminal switching "station to disable the corresponding transmitting line when voice currents are being received, means associated with said transmit-ting lines at each auxiliary station to disable the associated receiving line when the transmitting line is transmitting voice currents to said transmission medium, said lastmentioned means being sensitive to the weak initial portions of a voice wave before it approaches its peak value, means at each terminal switching station to transform the voice current volume received from a connecting line to a substantially constant predetermined volume so that the voice currents will arrive over the transmitting lines at the auxiliary stations with substantially constant volume, and means associated with the receiving line at each auxiliary station to disable the associated transmitting line when telephone currents are being received from sai transmission medium.

13. In a telephone system, a pair of terminal switching stations, a four-wire circuit comprising a transmitting line and a receiving line extending between each terminal station and an auxiliary station, a two-way submarine telephone cable extending between said auxiliary stations, means to connect said submarine cable to receive voice currents from the transmitting line of the four-wire circuit at each auxiliary station, means to connect said submarine cable to transmit voice currents to the receiving line of the four-wire circuit at each auxiliary station, means at said terminal switching stations to establish Cal stations to prevent the transmission of noise from connecting lines over the transmitting lines to said auxiliary stations" at all times when said transmitting llnes are not bemg' used for-voice transmission, means associated with said transmitting lines at each auxiliary station to disable the associated receivingline when the transmitting lineis transmitting voice currents to said cable, and means associated with the receiving line .at each auxiliary station to disable the associated transs mitting line when telephone currents arebeing received from said cable.

14. In a telephone'system, a pair of terminal switching stations, a four-wire circuit comprising a transmitting lineand a receiving line extending between each terminal station and an auxiliary station, a two-way submarine telephone cable extending between said auxiliary stations, means to connect said submarine cable to receive voice currents from the transmitting line of the four-wire circuit at each auxiliary 'station means to connect saidsubmarine cable to transmit voice currents to the receiving line at the four-wire circuit at each auxiliary station, means at said terminal switching stations to establish connections between the system and connecting lines, means at said terminal switching stations tovprevent the transmission of noise from connecting lines over the. transmitting lines to said auxiliary stations at all times when said transmitting lines are not being used for voice transmisslon, means associated withsaid transmitting lines at each auxiliary 'stationtodis able the associated receiving line when the transmitting line is transmitting voice currents to said'cable, said last-mentioned means being sensitive to the weak initial portions of a voice wave before it approachesits peak value, and means associated with the receiving line ateach auxiliary station to disable the associated transmitting line when telephone currents are being received from said cable.

1 5. Ina telephone system, a pair 'of terminal switching stations, a four-wire circuit comprising a transmitting line anda receiving line extending between each terminal station and an auxiliary station, the lines of said four-wirecircuits being laid in cable so as to be relatively free from noise due to external sources, atwo-way submarine telephone cable extending between saidauxil iary stations, means. to connect sald' submarine cable to receive voice currents'froin the transmittmg line ot-the four-wire circu t at each auxiliary station, means to connect said submarine cable to transmit voice j current-s to the receiving line of the four-wire circuit "at each auxiliary station, means said ter minalswitching stations to establishconneo tions between the system and connecting lines, means at said terminal switching stations to prevent the transmission of noise from connecting lines over the transmitting lines to said'auxiliary stations at all times when said vtransmitting lines are not being used for vo ce transmission, means associated with said transmitting lines at each auxiliary station to disable the associated receiving line when the transmitting line is transmitting voice currents to said cable, said lastmentioned means being sensitive to the weak initial portions of a voice wave before it approaches its peak value, and means associated with the receiving line at each auxiliary station to disable the associated transmitting line'when telephone currents are being received from said cable.

16, In a. telephone system, a pair of terminal switching stations, a four-wire circuit comprising a transmitting line and a receivng'line extending between each terminal station and an auxiliary station, a two-way submarine telephone cable extending between said auxiliary stations, means to connect said submarine cable to receive voice currents from the transmitting line of the tour-wire circuit at each auxiliary station, means to connect said submarine cable to transmit voice currents to the receiving line of the four-Wire circuit at each auxiliary station,

means at said terminal switching stations to establish connections between the system and connecting lines, means at said terminal switching stations to normally disable said transmittin lines, thereby preventing the transmission'of' noise thereover from connecting lines, voice-operated means associated with said'transmitting lines for rendering them operative, means associated with 'said'transmitting lines at each auxiliary station to disable the associated receiving line whenthe transmitting line is transmitting voice currents to said cable and means associated with the receiving line at each auxiliary station to disable the associated trans mitting line when telephone currents are being received from said cable.

17, In a telephone system, a pair of terminal switching stations, a four-wire circuit comprising a transmittingline and a receiving line extending between each terminal station and an auxiliary station, a two-way submarine telephone cable extending between said auxiliary stations, means to connect said submarine cable to receive. voice currentsfrom the transmitting line of the four-wire circuit at each auxiliary station, means to tour-wire circuit at each auxiliary station,"

means at said terminal switching stations to I establish connections between the system and connecting lines, means at said terminal switching stations to normally dlsable said transmitting lines, thereby preventing the transmission of noise thereover from connecting lines, VOlCG-OPelfltQd means associated with said transmitting lines for rendering them operative, means associated with said transmitting lines at each auxiliary station to disable the associated receiving line when the transmitting line is transmitting voice currents to said cable, said last-mentioned means being sensitive to the weak initial portions of a voice wave before it approaches its peal; value, and means associated with the receiving line at each auxiliary station to disable the associated transmitting line when telephone currents are being received from said cable.

18. in a telephone system, a pair of terminal switching stations, a four-wire circuit comprising a transmitting line and a receiving line extending between each terminal station and an auxiliary station, the lines of said touuwire circuits being laid in cable so as to be relatively free from noise from external sources, a two-way submarine telephone cable extending between said auxiliary stations, means to connect said submarine able to receive voice currents from the transmitting line of the tour-wire circuit at each auxiliary station, means to connect said submarine cable to transmit voice currents to the receiving line of the four-wire circuit each auxiliary station, means at said terminal switching stations to establish connections between the system and connecting lines, means at said terminal switching stations to normally disable said transmitting lines, thereby preventing the transmission of noise thereover from connecting lines, voice-operated means associated with said transmitting lines for renderin them operative, means associated with said transmitting lines at each auxiliary station to disable the associated receiving line when the transmitting line is transn'iitting voice currents to said submarine cable, said last mentioned means being sensitive to the weak initial portions of a voice wave before it approaches its peak value, and means associated with the receiving line each auxiliary station to disable the associated transmitting line when telephone currents are being received from said submarine cable.

19. In a telephone system, a pair of terminal switching stations, a tour-wire circuit comprising a transmitting line and a receiving line extending between each terminal station and an auxiliary station, a two-way submarine telephone cable extending between said auxiliary stations, means to connect said submarine cable to receive voice currents from the transmitting line of the tour-wire circuit at each auz-ziliary station, neans to connect said submarine cable to transmit voice currents to the receiving line of the four-wire circuit ateach auxiliary station, means at said terminal switching stations to establish connections between the system and connecting lines, means at said terminal switching stations to normally disable said transmit ting lines, thereby I ireventing the transmission of noise thereover from connecting lines, voice-operated means associated with said transmitting lines for rendering them ope 'ative, means associated with the receiving line at each teminal switching station to disable the corresponding transmitting line when voice currents-are being received, means associated with said transmitting lines at each auxiliary station to disable the associated receiving line when the transmitting line is transi'i'iitting voice currents to said cable, and means associated with the receiving line at each auxiliary station to disable the associated transmitting line when telephone currents are being received from said cable.

20. In a telephone system, a pair of terminal switching stations, a tour-wire circuit comprising a transmitting line and a receiving line extending between each terminal station and an auxiliary station, a two-way submarine telephone cable extending between said auxiliary stations, means to connect said submarine cable to receive voice currents from the transmitting line of the tour-wire circuit at each auxiliary station, means to connect said submarine cable to transmit voice currents to the receiving line of the four-wire circuit at each auxiliary station, means at said terminal switching stations to establish connections between the system and connecting lines, means at said terminal switching stations to normally dis- 1 able said transmitting lines, thereby preventing the transmission of noise thereover from connecting lines, voice-operated means associated with said transmitting line for rendering them operative, means associated with the receiving line at each terminal switching station to disable the corresponding transmitting line when voice currents are being received, means associated with said transmitting lines at each auxiliary station to disable the associated receiving line when the transmitting line is transmitting voice currents to said cable, said bust-mentioned means being sensitive to the weak initial portions of a voice wave before it approaches its peak value, and means associated with the receiving line at each auxiliary station to disable the associated transmitting line when telephone currents are being received from said cable.

21. In a telephone system, a pair of ter minal switching stations, a four-wire circuitcomprising a transmitting line and a receiving line extending between each terminal station and an auxiliary station, the lines oi? said four-wire circuits being laid in cable so as to be relatively free from noise from exthe receiving line of the four-wire circuit at each auxiliary station, meansat said terminal switching stations to establish connections between the system and connecting lines, means atsaid terminal switching stations-to normally disablesaid transmitting lines, thereby pre venting the transmission of noise thereover from connecting lines, voice-operated means associated with said transmitting lines .for rendering them operative, means associated with T the receiving line 'ateach terminal switching station to disable the corresponding transmitting line when voice currents are being received, means associated with said transmitting lines at each auxiliary station to disable the associated receiving line when the transmittinglineuis transmitting vvoice currents tosaid submarine cable, saidzlastmentioned means being sensitive to the weak initial portions of a voice wave before it approaches its peak volume, and means asso-v ciated with the receiving line at each auxil-. iary station to disablethe associated transmitting line when telephone currents are being received from said submarine cable:

22.1n'atelephone system, a pair'of'terminal switching'stations,a four-wire circuit comprising a transmitting line and areceiving line extending between each terminal station and an auxiliary station, a two-way submarine telephone cable extending between said auxiliary stations, means to connect-said submarine cable toi receive voice currents from the transmitting line of theffour-wire circuit at eachauxiliary station, means to connect' said submarine cable to transmit voice currents to the receiving line of the four-wire circuit at each auxiliary station, means at said "terminal switchingstations to establish connections between the system and connecting lines, means at. saidterminal switching stations "to prevent the transmission of: noise from ,connectinglines over the transmitting lines to said auxiliary stations at all times when said transmitting lines are not-being used for voice transmission, means associated with said transmitting lines at each auxiliary station to .disable the associated receiving line when the transmitting line istransmitting voice currents to" said able, said lastmentionedrmeans being sensi: tive to the weak initial portions of a'lvoice wave before it" approaches its peak value, means at each terminal switching station to transform the [voice currentvolume received from a connecting lineto a substantiallyconstant predetermined volume so that the voice currents will arrive over the transmitting lines at the auxiliary stations with substantially constant volume, and means associated with the receiving line at each auxiliary station to disable the associated transmitting line when telephone currents are being received from said cable.

23. 'Ina telephone system, a pair of terminalswitching stations, a four-wire circuit comprising a transmitting line and a receivingline extending between each terminal station and an auxiliarystation, a two-way submarine telephone cable extending between said auxiliary stations, means to connect saidsubmarine cable to, receive voice currents from the transmitting line of thefour wire circuit at each auxiliary station, means to connect said submarine cableto transmit voice currents to the receiving line of the four-wire circuit at each auxiliary station, means at said terminal switching stations to establish connections betweenthe systemand connecting lines, vmeans at said terminal switching stations to normally-disable said transmittingllines, thereby preventing the transmission of noise thereover from connecting lines, voice-operated means associated with said transmitting lines for rendering'them operative, means associated with said transmitting lines at each auxiliary station to disable the associated receiving line when the transmitting line is transmitting voice currents to said cable, said last-mentioned means being sensitive to the weak initial portions of a voice wave before it ap; proaches its peak value, means at each terminal switching station to transform the voice current volumereceived from a connecting line to a substantially constant pre determined volume so thatthe voice currents will arrive over the transmitting lines at the auxiliary stations with substantially constant volume, and means associated with the receiving line at each auxiliary station to disable the associated transmitting line when telephone currents are being received from saidcable. J 5 V v j,

24. In a'telephone system, a pair of ten minal switching stations, a four-wire circuit comprising a transmitting line and a receiving line extending between each terminal station and an auxiliary station, a two-vwaysubmarine telephone cable extending between said auxiliary stations, means to connect said submarine cable to receive voice currents from the transmitting linefof the four-wire circuit at each auxiliarystation, means to connect said' {submarine cable to transmit voice currentsto the receiving line of-the four-wire circuit at each auxiliary station,

means atsaid terminal switching stations to establish connections between the sys tem and connecting lines, 'means'at said terminal switching stations to normally dis able said transmitting lines, therebypreventing the transmission of noise thereover from connectin'g lines, voice-operated means associated with said transmitting lines for ren dering them operative, means associated with the receiving line at each terminal switching station to disable the corresponding trans mitting line when voice currents are being received, means associated with said transmitting lines at each auxiliary station to disable the associated receiving line when the transmitting line is transmitting voice currents to said cable, said last-mentioned means being sensitive to the weak initial portions of a voice wave before it approaches its peakvalue, means at each terminal switching station to transform the voice curren't'volume received from a connecting line to a substantially constant predetermined volume so that the voice current-s will arrive over the transmitting lines at the auxiliary stations with substantially constant volume, and means associated with the receiving line at each auxiliary station to disable the associated transmitting line when telephone cur rents are being received from said cable.

25. In a telephone system, a pair of terminal switching stations, a four-wire circuit comprising a transmitting line and a receivingline extending between each terminal station and an auxiliary station, a two-way submarine telephone cable extending between said auxiliary stations, said cable having an impedance which includes a substantially reactance component so that a relaxing current will be discharged by the cable upon the ces sation of applied signaling energy, said cable also being of such dimensions that its attenuation will be so great that received signaling currents transmitted over the cable will be small as compared with said relaxing current, means at each auxiliary station to connect the terminals of said cable to said fourwire circuit so as to receive signaling currents from the transmitting line and trans mit signaling currents to the receiving line, and means associated with said submarine cable to reduce the effect of the relaxing current in the receiving line of each four-wire circuit.

26. In a telephone system, a pair of terminal switching stations, a four-wire circuit comprising a transmitting line and a receiving line extending between each terminal station and an. auxiliary station, a two-way sub marine telephone cable extending between said auxiliary stations, said cable having an impedance which includes a substantial reactance component so that a relaxing currentwill be discharged by the cable upon the cessation of applied signaling energy, said cable also being of such dimensions that its attenuation will be so great that received signaling currents transmitted over the cable will be small as compared with said relaxing current, a hybrid coil at each auxiliary station to con nect the terminals of said cable to said fourwire circuit so as to receive signaling currents from the transmitting line and transmit signaling currents to the receiving line, and a balancing network so associated with said submarine cable and the hybrid coil as to reduce the efilect of the relaxing current from the submarine cable in the receiving line of each four-wire circuit.

27. In a telephone system, a pair of terminal switching stations, a four-wire circuit comprising a transmitting line and a receiving line extending between each terminal station and an auxiliary station, a two-way submarine telephone cable extending between said auxiliary stations, said cable having an impedance which includes a substantial reactance component so that a relaxing current will be discharged by the cable upon the cessation of applied signaling energy, said cable also being of such dimensions that its attenuation will be so great that received signaling currents transmitted over the cable will be small as compared with said relaxing current, a hybrid coil at each auxiliary station to connect the terminals of said cable to said four-wire circuit so as to receive signaling currents from the transmitting line and transmit signaling currents to the receiving line, and a balancing network associated with said hybrid coil and said submarine cable at each auxiliary station, said balancing network having suchimpedance as to produce a relaxing current of substantially the same initial value and having the same decrement as the relaxing current of the submarine cable whereby the eitectivc relaxing current in the receiving line of each tourwire circuit will be small.

28. In a transmission system including an intermediate section connected at each end with connecting sections, means at each end of said intermediate section to block the transmission of waves in either direction while waves are passing in the opposite direction, and means at points of the connecting sections away from the end of said intermediate section to block the transmission of waves toward the intermediate section while waves are passing in the opposite direction, said connecting sections being so arranged as to permit at all times the passage of waves past the points where said last mentioned means are located in a direction away from the intermediate section.

29. In a telephone system, a pair of terminal switching stations, a four-wire circuit comprlsmg a transmitting line and a receiving'line extending between each terminal station and an auxiliary station, a transmission medium between said auxiliary stations, means at said terminal switching stations to establish connectionsbetween the system and connecting lines, means at said terminal switching stations to prevent the transmis- 

